1. Field of the Invention
This invention relates to apparatus and methods for reading data on storage media such as magnetic tape.
2. Background of the Invention
As the storage capacities of modern tape cartridges are pushed higher and higher, it is becoming increasingly difficult to read data from magnetic tape. Small variations in manufacturing or variations in temperature, humidity, or head-to-tape interfaces can cause changes in the read-back signal which may cause traditional recording channels to fail to detect data. Modern tape drives are able to handle this variability by being adaptable. Based upon the signals read from tape, the read-detect channel of the modern tape drive can adjust the equalization of the read-back signal to improve the signal-to-noise ratio. It can also compensate for head asymmetry or modify data detection parameters to improve the detection reliability.
One of the problems with the adaptability discussed above is that sometimes the read-detect channel does not converge to an optimal configuration. Instead, it may diverge from an optimal configuration such that data can no longer be detected. With otherwise good media, head, and tape path, the channel may adapt in a manner that renders the data useless. This problem tends to get worse as the storage density of tape increases.
Divergence of the equalizer is the primary cause of read-detect channel instability. In many cases, this divergence may be controlled by fixing a certain number of tap coefficients in the adaptable finite-impulse-response (FIR) equalizer (also known as a FIR filter). For example, in a typical FIR equalizer that includes seventeen taps, four tap coefficients out of seventeen may be fixed. If the correct tap coefficients are fixed, then the FIR equalizer will be stable and the FIR equalizer will converge to an optimal configuration (assuming that the initial configuration of the FIR equalizer was reasonable). If the wrong tap coefficients are fixed, then the FIR equalizer will not converge and the equalization will eventually get so bad that the data read will not be usable.
In the FIR equalizers of many current tape drives, four adjacent tap coefficients out of seventeen are fixed. In such drives, the initial configuration (i.e., tap coefficients) of the equalizer may be determined using a calibration procedure. The largest tap coefficient of the seventeen may then be fixed. Two tap coefficients on one side of the largest tap coefficient and one tap coefficient on the other side of the largest tap coefficient may be fixed. The side with the two fixed tap coefficients is typically selected to be in the direction of the next largest tap coefficient of the seventeen coefficients. This methodology has worked well historically. However, as tape linear densities have increased and recording properties of the storage media have changed, this methodology increasingly results in divergent FIR equalizers.
In view of the foregoing, what is needed is an improved methodology to select which tap coefficients of a programmable FIR equalizer to fix. Ideally, such a methodology will allow FIR equalizers to more consistently converge to an optimal configuration that improves, rather than worsens, the signal to noise ratio.